Inner
structure of a key top is designed to let the spring bend toward
designated direction only inside a key unit. When a
spring completely bends ( buckles ) , it will hit actuator
and generates clicking sound and at the same time a platter ( or
hammer ) at
the end of the spring will hit and press membrane sheets (
membrane sheets are covered by a black rubber sheet . You can't see membrane sheets in above
pictures. )

Springs
and hammers
Coil springs and hammers used in model M2 ( including 42H0468 ) are
identical with those used in Model M.
Some models have black hammers while the others have white
half-translucent hammers.
Those coils and hammers can be interchangeable among all of model M
( of course except rubber spring type M ).
Japanese readers are advised that coil springs in 5576-A01/003 are
different from those used in Model M. They are shorter than
Model M springs.

Pictures
below show deference of an actuating rocker (hammer) of Model M and
Japanese A01

L; Model
M
R; Japanese A01

L; Model
M,
R; Japanese A01

L; Model
M,
R; Japanese A01

Bottom view

Upper view

Upstanding post on the upper surface of a
rocker

Is
there any rule to set a spring to a hammer ?
As far as I can see, every springs set on each hammers with a starting
point of coil spring locates within 180 to 270 degrees. Don't know
if there is any reason for it. If I set a spring in
different position, buckling error occurs frequently.

How
to reduce Buckling sound
If you feel your M is too loud when you type and you want to reduce
buckling sound without changing your M to a cheapo membrane keyboard .Use silicon grease. Pull out key
assembly and paste a little dose of silicon grease around the wall of
barrel and buckling springs. You can control sound level with the volume of
grease.
Spray type silicone grease is also easy to
use.
............And actually IBM provided such a
variant of Model M.

Model
F ; Buckling spring technology with capacitive switches
Model F was introduced together with the IBM PC. Key action is US
patented IBM Buckling technology and the switching system is so called
capacitive contacts switch. Capacitive switch does not require any
physical contacts between two terminals. Specially designed
electrical components detects changing of capacitance between
terminals and at some point
of capacitance, switch turns on. Model F uses plastic made
platters ( hammer lever ? ) to change capacitance of each switches, When a platter hits main PCB,
capacitance between two terminals on the PCB placed below a platter
increases and immediately a switch turns ON.
Capacitive switches made by other manufacturers such as Keytronic, BTC
and TOPRE ( a Japanese manufacturer which still makes very expensive
keyboards with capacitive switches ) do not generate clicky
sounds. Only IBM's model F with buckling system makes
louder clicking sounds when platters hit main PCB directly. Those who like IBM model
M may like IBM model F but those who does not like model M because of
its buckling sounds definitely dislike model F .
Because Model F is much louder than Model M when we
type. I like model F so much. Model
F AT 84 key keyboard is the one you can use with usual PCs without
any modification. You just need an AT to PS/2 cable to use the
keyboard.

English
explanation about Capacitive switch is available in this
web page.
The author misunderstand about mechanical switch because the author
introduced buckling mechanism of model F as an example of
mechanical switch. As far as Capacitive switch
concerns, the explanation seems to be correct..

Please
examine U.S. Patent No.4,188,611
for further detailed information.

Glass
fiber PCB is attached on the curved lower steel plate. PCB
itself is not flat but is also in a wide curve.

Looking
from the bottom side of the lower steel plate

Left
side; Comparator?
A large DIP module might be a controller. It 's marked as
"8310354 8332, PI P A 998741, L3374976, Intel "

Back
View of a controller PCB

Some
kind of coating covers surface of the main PCB including every
terminals of signal line ( large square parts ) .
Look carefully a close look of terminals. You can identify no
cupper surface is exposed on the main PCB.

Wow!
Such a big plater ( blade, hammer lever , I don't know how
IBM named it. )
The lever is made of carbonized plastic.
When the key press downward and the hammer meets terminals on
the PCB, capacitance between the terminals are increased
and then a switch turns ON. i don't know detailed mechanism.

Every
actuators can be removed from metal base plate ( upper plate
).

Do
not pull out a space bar from this keyboard, XT 83 key
keyboard and AT 84 key keyboard. Stabilizing bar
is set underneath the upper metal plate and you can't place it properly
unless you disassemble the keyboard .
That's why I can provide above pictures. I knew that, but I just
forgot about it when I pulled out keys to make them
clean......

Front view of P/N 5640991

Keys except text/data keys may be
placed wrong position.
Forgot to take pictures before I pulled
out all the keys :-)

1392993
K26-1*1 EC 528682 3787, dimension
170mm x 50mm
Pulled from Model M 1391401 manufactured in 22SEP1987.
Almost same with 1389989 above. There are 2 capacitor packs at the
position of CP1 and CP2 below J4. Except these two
register packs, This PCB seems almost same with
1389989. ( I'm not so sure whether "CP"
means capacitor
packs or not. )

Looks like almost same with 1389989 1392993 except missing 2 brown capacitor
packs below J4. There are
solder pads at the position CP1 and CP2 . Main control
chip is different from above 2 PCBs.

This PCB is used in Gray color model M keyboard with a black square logo
plate , Part number 1388032, ID 1001377, Date 02OCT87, Plt J3
The keyboard doesn't work with a usual PC ( I mean Wintel PC not a
genuine PS/2, don't know if if works with a PS/2 ) . I don't know
if those missing CP1/CP2 blocks have some effects on this issue.
Or if the difference of controller chip P/N causes the problem...
When I swap the control PCB to a 1392993, the keyboard works
properly.

2004.06.27
Examined DC level of data/clock line and found each lines are something around 2.1V while others keyboards are around 4.8V.
Then I tried data/clock line modification adding 2x 4.7K ohm
registers according to IBM
PS/2 Keyboard modification page . Result ; No luck.. Three LEDs flashed repeatedly and PS/55E gave
301 error again. Conclusion : Model M P/N 1388032 with a controller PCB P/N
1351523 might be a proprietary keyboard for some special
system. Most possibly Keyboard controller is not
compatible with usual PCs. ( Or mine has a defective
controller PCB )

U1/U3
: 2392161 U2 : 1393295Connector at J2 is missing, instead there is J8
for film cable connector for LED block

Seems
to be a popular type of control PCB used in earlier type of Model M between
1988 and 1990 .
Main difference is connection between a control PCB and LED block. Each
PCBs are connected via film cable instead of wires with 4 pin headers.
Variations of this PCB in smaller dimension can be seen in Louis's
page.

PCB is
pulled from a Model M P/N 59G7980 by Lexmark with drain holes, blue logo on a
gray round plate, manufactured 02FEB1993.
Film cable for LED block is combined with matrix lines and connected to J3 on the PCB together with
key matrix lines. This PCB is made in HongKong.

Bottom
left : Clock , to
the Yellow cable
Bottom center : GND , to the White cable
Bottom right : DATA, to the Red cable
Upper right : +5V , to
the Black cable

Keyboard
p/n 1390680 has very funny characters at the position of Numeric
Keys. Numeric keys are available at the upper row of text/data
keys ( below Function keys ) .
This Control PCB does not work on with
Windows PC.

Stabilizing
RodIn addition to Space Bar, early models of 1391401 have
stabilizing rod beneath both "Enter" and
"+" key at numpad section. This
design was already seen in 1390121 and 1390130 sold together with late
XT and AT computers respectively, one year earlier than appearance
of 1391401 and PS/2 computers .

1390121

1390131 Sorry
can't find clean one

1391401 '87 made

13912401 '88 made

1392598 ( Not
1391401 but 87 made Model M )

1391401 '88 made in UK (
US layout )

Seems like IBM changed design at some
point in 1989

Some of 1391401 made after 1989

Model
M can't be recognized when connected to some modern mother board

It
is reported byMr. John Szybowski that model M can't be used
properly when it is connected to some newer mother board with faster
processor. This problem can be solved according to John's method.
Go here ; IBM
PS/2 keyboard modification page( stored at web.archive.org)
.